In an electric device that includes a plurality of conductor planes, electromagnetic waves are generated. Such electromagnetic waves are generated by a magnetic field induced by a current flowing into a digital circuit when the circuit is switched, or by an electric field induced by a voltage change that occurs when the circuit is switched. Such electromagnetic waves become electromagnetic noise that propagates in a parallel-plate transmission line formed of conductor planes. Such electromagnetic noise results in such problems as destabilized operation of other circuits and deteriorated performance of a wireless function of the device. In other words, establishment of a technique that suppresses electromagnetic noise allows stabilization of circuits and improvement in performance of a wireless function of the device.
Examples of methods that have been used to solve the above problems include: a method in which to insert a decoupling capacitor between conductor planes; and a method in which to avoid production of a large insular conductor plane. These methods, however, involve the following problems. In the method using a decoupling capacitor, unavoidable parasitic inductance of a capacitor makes it difficult to obtain a self-resonant frequency at a high frequency such as several hundred hertz. For this reason, the method using a decoupling capacitor can only be applied to frequencies up to about several hundred megahertz, and is not appropriate for high frequency ranges (for example, the 2.4-GHz range and the 5.2-GHz range), which have recently been used in wireless communication. The method in which to avoid production of a large insular conductor plane is based on the principle that the resonance frequency of conductor planes is shifted to a higher frequency by making conductor planes smaller. In practice, however, conductor planes having the same potential need to be connected in series to each other. For such connection, a connecting part between the insular conductor planes needs to be made thin. If the connecting part is made thin, the self inductance of that part increases, and consequently, a large voltage drop occurs at the time when a current flows when switching is performed. Thus, there is a practical limit in reducing conductor planes in size.
Examples of methods for solving the above problems include a method disclosed in Patent Document 1. Each of the structures disclosed in Patent Documents 1, 2, and 3 is a structure having the electromagnetic bandgap (EBG) property (hereinafter, referred to as the EBG structure), and is intended to suppress propagation of electromagnetic wave noise between power supply planes. The use of an EBG makes it possible to provide an electromagnetic-noise suppressing effect to the GHz ranges. Furthermore, unlike the method in which to provide conductor planes in the form of separate smaller islands, these methods involve no special processes on a power supply plane, and hence do not increase the self-inductance of the power supply plane.